Method of purifying zinc hydrosulfite solutions



United States Patent 3,216,791 METHOD OF PURIFYING ZINC HYDROSULFITE SOLUTIONS Virgil L. Hansley and Louis F. Moorrneier, Cincinnati,

Ohio, assignors to National Distillers and Chemical Corporation, New York, N.Y., a corporation of Virginia No Drawing. Filed Apr. 11, 1962, Ser. No. 186,658 6 Claims. (Cl. 23-116) This invention relates to a method of purifying zinc hydrosulfite. More particularly, it relates to a method of reducing the quantity of heavy metal impurities, principally iron, in zinc hydrosulfite solutions.

Certain processes in which zinc hydrosulfite is employed commercially require a minimum of contamination from heavy metals. For example, one important commercial process utilizing zinc hydrosulfite is the production of sodium hydrosulfite according to the following reaction:

In order to have an economic process for sodium hydrosulfite the greater part of the zinc must be recovered. Generally this is accomplished by calcining the by-product zinc carbonate to a white pigment grade of zinc oxide. Certain heavy metal impurities, e.g., iron, nickel, and copper, if present in the zinc in excessive quantities, carry over to the zinc oxide in sufiicient quantities to impart color and limit the value of the zinc oxide as a pigment. Similarly, the value of zinc hydrosulfite as a bleaching agent, e.g., for groundwood, paper and pulp, and other cellulosic materials, is severely limited by contamination with color-imparting heavy metals.

Heavy metal contamination of zinc hydrosulfite and zinc oxide is commonly avoided by employing a high grade of distilled zinc dust in preparing zinc hydrosulfite by the following reaction:

Use of commercial, distilled zinc dust, however, is not only relatively expensive, but is attended by fire and explosive hazards. A more preferred method is that described in pending U.S. patent application Serial Number 133,323, filed August 23, 1961, wherein relatively impure grades of zinc, in the form of sodium-zinc alloys, are employed. Such impure grades of zinc include galvanizers zinc dross and die casters waste. While use of these forms of impure zinc leads to a more efficient, more economical, and safer process for producing zinc hydrosulfite, the zinc hydrosulfite produced therefrom often contains undesirable quantities of color-producing heavy metal contaminates, particularly iron.

. Accordingly, it is an object of this invention to provide an efiicient and economical method for the purification of zinc hydrosulfite solutions from heavy metal impurities, especially iron.

Another object of this invention is to provide an economical method for producing purified zinc hydrosulfite suitable for use in the commercial manufacture of sodium hydrosulfite and by-product zinc oxide suitable for use in pigments, starting with zinc which contains objectionable quantities of heavy metal impurities.

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Still another object of this invention is to provide a means of using impure zinc for the production of zinc hydrosulfite suitable for various commercial uses.

Other objects will become apparent from the ensuing detailed description of the invention.

Impure zinc hydrosulfite solutions are obtained by reaction of contaminated zinc feed material such as zinc dust or zinc-sodium alloy with sulfur dioxide in aqueous solution at a temperature between about 0 and 50 C., and preferably between about 30 and 35 C. The pH must be maintained between 3.5 and 5.5 because at higher values the reduction tends to be slow, while at values below about 3.0 Zinc hydrosulfite solutions tend to be unstable.

The sodium-zinc alloy can be made by any convenient method. Most commonly it is prepared by melting together sodium and zinc e.g., ingot zinc, galvanizers zinc dross, die-casters zinc waste, at just above the melting point of the zinc sodium alloy. T o obtain pieces of the desired size, (1) the material may be cast into slabs which can be crushed or ground upon cooling, or (2) the crystallizing mass may be agitated through its melting temperature in an inert atmosphere.

The amount of sodium in the sodium-zinc alloy may vary widely, but in general a sodium content of about 0.5 to about 4 weight percent is used and a sodium content not over about 3.1 percent is preferred. An alloy containing up to about 2.0% of sodium is sufiiciently brittle to permit easy grinding. Actually, the true alloy corresponds to the formula Zn Na or Zn Na with more sodium being carried in solid solution up to the point of 3.54% where the free sodium phase begins to be evident. A higher sodium content results in a product which is soft, malleable, and which can be ground only with difficulty because of the presence of a sodium-rich phase. Also, an alloy having a sodium content higher than about 4% tends to react with oxygen and moisture in the air, resulting in an alkaline, damp material when in contact with moist air.

The preparation of the zinc hydrosulfite from zincsodium alloy may be effected in any suitable reaction vessel adapted for carrying out such a reaction, such as a static bed, a slowly agitated bed, e.g., a rotating drum reactor, or a recirculation system in which the whole reduction mixture is carried through a cooling coil to maintain proper temperature control, and from which product can be withdrawn as the reaction proceeds in a continuous manner under steady, stable reaction conditions.

In accordance with this invention, it has now been found that zinc hydrosulfite solutions, and particularly the impure zinc hydrosulfite solutions prepared by the above described process, can be purified from objectionable metallic impurities by contacting such solutions with the zinc salt of a cation exchange resin under ambient conditions.

These ion-exchange resins are generally high molecular weight polymers with ionic .groups incorporated in the lattice as integral parts of the polymeric structure. Cation exchangers contain carboxylic, phenolic, phosphoric, or sulfonic groups with an equivalent amount of cations. The cation exchange resins particularly suitable for the purpose of this invention include Dowex 50, Dowex 50X, Dowex 50W-X8, Permutit Q, Amberlite IR-l20, Permutit, and others. The Dowex, Permutit Q, and Amber- 3. lite IR-120 resins are polystyrene polysulfonates which are obtained by polymerization of styrene and divinylbenzene and subsequent sulfonation. Several producers manufacture sulfonated styrene type of resins as cation exchange resins under other trademarks. Permutit is a synthetic zeolite of the approximate composition NA2O A1203 inches long and inch in diameter filled with zinc salt of a sulfonated styrene resin (Dowex SOW-XS) over a period of 40 minutes. The purified zinc hydrosulfite solution showed essentially the same reducing power as the unpurified solution.

The purified zinc hydrosulfite solution and the unpuri fied zinc hydrosulfite solution were separately converted into solid sodium hydrosulfite and solid zinc carbonate. This was accomplished by treating a strong to aqueous solution of zinc hydrosulfite with sodium carbonate and then filtering off the very insoluble hydrated zinc carbonate which was formed. The sodium hydrosulfite was then crystallized from the aqueous solution as a dihydrate. The sodium hydrosulfite di'hydrate was then dehydrated with methanol and salt to yield the coarse, easily dried, anhydrous sodium hydrosulfite. The following table shows the spectographic analyses of the products with respect to iron, nickel, aluminum and copper.

T able ZnS O; Solution 7 ZnCO; Solids Na S O; Solution Na S O Solids Element Untreated After Untreated After Untreated Alter Untreated After Treatment Treatment Treatment Treatment Fe 4,100 p.p.rn 200 ppm--- 900 ppm 200 p.p.m 3,200 ppm"- 200 p.p.m 2,000 ppm- 200 p.p.m. Ni 45 1n 10. 2s 10 3% 10, Al 25 n 70 an 4 4 a 1 u 6 1. 12. 4 1 l l 1. Color Yellow-green, Colorless Yellow-brown White Light yellow Colorless Light tan White.

*Fe was held in solution up to this stage as iron-oxalate complex; otherwise it would have been largely precipitated with the zinc carbonate This shows that even Na s O solutions containing appreciable zinc sulfate is used. The amount of saturated zinc salt solution necessary to effect conversion of the resin to its zinc salt is determined by the resin capacity of the particlar cation exchange resin used. Producers of commercial exchange resins state the resin capacity of their product, i.e., the amount of resin needed for en equivalent quantity of cation, in this case Zn The reducing power residing in the hydrosulfite anion is substantially unaffected on passing through the resin. The purified hydrosulfite is sufliciently pure for commer-- cial uses. For example, when the zinc hydrosulfite is converted to sodium hydrosulfite by treating a strong (20-30%) aqueous solution of zinc hydrosulfite with caustic soda or sodium carbonate, the decomposition rate of the sodium hydrosulfite will not be materially increased and the zinc carbonate will have a white color suitable for the manufacture of pigment grade zinc oxide.

When the zinc charged cation becomes loaded with metallic impurities it can be regenerated (1) by eluting with a saturated solution of zinc sulfate or (2) by converting the resin into its acid form, for example, by eluting with sulfuric acid, followed by eluting with a saturated solution of zinc sulfate. After washing out the excess zinc sulfate solution, the resin zinc salt is again ready for use.

The following examples illustrate methods of carrying out the present invention, but it is to be understood that these examples are given for purposes of illustration and not of limitation.

EXAMPLE I 2.5 gram atoms of impure zinc, containing 0.98 percent of iron and alloyed with 1.87 percent of sodium, was added as a powder gradually to a solution of 4.0 moles of sulfur dioxide in 1200 ml. of air-free water over a period of 46 minutes. The pH of the system was maintained between 2.7 and 4.9 and the temperature was held between 29 and 31 C. Upon completion of the reaction, the solids present (mainly unchanged zinc) were removed by centrifuging.

One-half of the resulting zinc hydrosulfite solution so obtained was purified by passing through a column 48 alkali solubilized iron can be effectively purified of iron by the present process.

It can be seen from the table that the removal of iron is suflicient to eliminate color from the hydrosulfite solutions as well as from the sodium hydrosulfite and zinc carbonate solids. Consequently, the value of the hydrosulfites as bleaching agents and of zinc oxide pigment produced from the zinc carbonate is greatly enhanced. It also is seen from the table that the zinc salt of a cation exchange resin also may effect removal of heavy metals other than iron, e.g., nickel, aluminum, and copper.

EXAMPLE II 2.5 gram atoms of impure zinc, containing 0.96 percent iron and alloyed with 2.22 percent of sodium, was added in powdered form gradually to a solution of 4.0 moles of sulfur dioxide in 1200 ml. of air-free water over a period of 57 minutes. The pH of the system was maintained at 4.4 to 4.9 and the temperature at 27 to 31 C. The unreacted solid remaining at the completion of the reaction was removed by centrifuging.

One-half of the above zinc hydrosulfite solution was then purified by passing through a column 48 inches long and inch in diameter filled with a zinc salt of a high capacity commercial water softening resin of the zeolite type over a period of about 40 to 60 minutes. The zinc hydrosulfite solution so purified had essentially the same reducing power as the unpurified zinc hydrosulfite solution.

The purified zinc hydrosulfite solution and the unpurified zinc hydrosulfite solution were separately converted into solid sodium hydrosulfite and solid zinc carbonate according to the method of Example I.

The dried by-product zinc carbonate which was obtained from the treated portion of zinc hydrosulfite solution was white in appearance While the zinc carbonate obtained from the untreated portion was yellow-brown in color. Spectrographic analysis showed 4000 p.p.m. of iron in the zinc carbonate obtained from the untreated zinc hydrosulfite solution, and only 500 ppm. of iron in the zinc carbonate obtained from the treated solution. The latter sample also exhibited lower contents of both nickel and aluminum.

While particular embodiments of this invention are shown in the above examples, it Will be understood that the invention is subject to variations and modifications without departing from its broader aspects.

What is claimed is:

1. A method for purifying zinc hydrosulfite solutions from heavy metal contaminants which comprises contacting said zinc hydrosulfite solutions with a zinc containing cation exchange resin.

2. The method of claim 1 wherein the cation exchange resin is a sulfonated polystyrene resin.

3. The method if claim 1 wherein the cation exchange resin is a zeolite resin.

4. A method for removing iron impurities from an tacting said impure zinc hydrosulfite solution with a zinc containing cation exchange resin, and recovering purified zinc hydrosulfite.

5. The method of claim 4 wherein the cation exchange resin is a sulfonated polystyrene resin.

6. The method of claim 4 wherein the cation exchange resin is a zeolite resin.

References Cited by the Examiner UNITED STATES PATENTS 1,975,443 10/34 Bazlen et al. 23-416 2,653,089 9/53 Brooke 21038 impure zinc hydrosulfite solution which comprises con- 15 MAURICE A. BRINDISI, Primary Examiner. 

1. A METHOD FOR PURIFYING ZINC HYDROSULFITE SOLUTIONS FROM HEAVY METAL CONTAMINANTS WHICH COMPRISES CONTACTING SAID ZINC HYDROSULFITE SOLUTIONS WITH A ZINC CONTAINING CATION EXCHANGE RESIN. 